Optical fiber networks are increasingly important for the distribution of telephonic signals. One architecture of current interest for an optical fiber communication system is conveniently described with reference to FIG. 1. That architecture includes a fiber-optic terminal system (FOTS) 10 located at a central office (CO) 25. The FOTS includes receivers 20 for intercepting optical fiber transmissions coming into the CO, and transmitters 30 for initiating optical fiber transmissions which exit the CO. The optical fibers 40 which communicate directly with the FOTS are typically fibers internal to the CO. These fibers need to be interconnected with fibers 50,55 which exit the CO; i.e., with fibers in the outside plant. The interconnections are performed through a lightguide cross-connect frame (LGX) 60. A typical LGX is a sheet-metal frame which houses many fibers having ends which are adapted, by appropriate connectors, to be manually interconnected. Typically, those fibers 50 that carry outbound transmissions from the CO to the outside plant are distinct from those fibers 55 that carry inbound transmissions from the outside plant to receivers at the CO. Several thousand outside-plant fibers may enter and exit a CO which serves a metropolitan area.
Turning now to FIG. 2, an active star architecture may be used to distribute transmissions between a group of fibers 70 extending over typical lengths of 1-50 km from the CO to respective remote terminals (RTs) 80 and a much larger group of fibers 90 extending from the RTs to distant terminals (DTs) 100. The DTs may, for example, be located at individual residences, or they may be situated at curbside enclosures each of which feeds several residences. Each RT may include a FOTS for relaying optical fiber transmissions between fibers that carry signals into the FOTS and fibers that carry signals out of the FOTS. Each RT may also include an LGX for performing interconnections between the FOTS input and output fibers and the outside plant fibers which communicate with the RT.
In order to assure that the network is operating properly, it is necessary to perform tests on the network and to monitor transmissions in the fibers of the network. In particular, it is important to provide access points for testing and monitoring the fibers that emerge from the CO of FIGS. 1 and 2, and the downstream fibers emerging from the RTs of FIG. 2. The fibers at each location that are to be monitored may number in the hundreds, or even in the thousands. Monitoring of such a large number of fibers by active techniques may be prohibitively expensive. Moreover, active monitoring generally involves undesirable service interruptions. Consequently, it is desirable to provide one or more passive components which include access points for monitoring which can be used during ordinary service. However, space is likely to be limited at the CO, and even more limited at the RTs. Limitations on space may limit the number of fibers that can be monitored by conventional, passive components. Therefore, where it is necessary to monitor a very large number of fibers, it may be necessary to use miniaturized passive components for testing and monitoring of the network fibers. Miniaturizable, passive components which can be incorporated in an optical communication system, and which provide access to the fibers for testing and monitoring, are described below.